This invention relates to data processing apparatus and methods. More particularily, it relates to such apparatus and methods wherein data is processed under the control of a program comprising a sequence of instruction words. The apparatus carries out the processing defined by each instruction word and then furnishes an output word, which generally constitutes the address of the next subsequent instruction word but may also comprise computed data and the address to which the computed data is to be furnished.
The output word furnished by the equipment of the present invention may also be utilized to control the operation in automated machine tool or production equipment, in data input and output devices, etc.
Data processing equipment of this type is generally known as microprocessors. These comprise a read-only memory storing the instruction words, a central processing unit, and a random access memory. Of course, additional external storages may be associated therewith.
Under modern production techniques, microprocessors generally are built as integrated circuits. In such integrated circuits the space occupied by the actual circuitry is extremely small, the overall space required by the equipment being mainly a function of the number of connections which must be made to each unit. In attempting to minimize the cost of, and the space occupied by, a microprocessor, it is therefore essential to minimize the number of external connections required, rather than the number of actual circuit components. Further of course, the microprocessor according to the present invention should be as versatile, as possible, that is, its application should not be restricted to a particular application. This requires that the output words be usable without further adaptation in a variety of external equipment.
Since the output words of apparatus of the present invention are frequently used to address one of a large number of storage locations in the external data processing system, the number of bits in the output words has to be relatively large. One possibility of furnishing long address words with relatively little equipment is to generate the first part of the address word, store this in an external register, furnish the second part of the address word, store this in another register, and then output the two parts simultaneously. This requires three or four individual steps so that a relatively long time is required for outputing such addresses. Further, an additional external register, having a number of places corresponding to the total number of bits in each output word, is required. This in turn increases the cost of such an arrangement.
One possibility of saving terminals is to use each terminal both as an input and as an output terminal. However, such arrangements have very definite drawbacks especially insofar as testing the equipment is concerned. Further, the terminals must of course be decoupled from each other which requires additional circuit elements such as diodes or transistors. Testing of the equipment must then proceed separately for the input function and the output function, thereby increasing the manufacturing time.